Frequency mixers may be included in many types of electronic systems. For example, frequency mixers in radio systems down convert a received radio frequency (RF) signal by combing the RF signal with a local oscillator (LO) signal. The combination of the RF signal and the LO signal yields an intermediate frequency (IF) signal, which has a frequency corresponding to a difference between the RF and LO signals.
FIG. 1 is a block diagram of a known mixer 100, which includes LO-port 110 for inputting a LO signal, RF-port 120 for inputting a received RF signal and IF-port 130 for outputting an IF signal. The mixer 100 also includes a single grounded-source field effect transistor (FET) 112, which may be a gallium arsenide field-effect transistor (GaAsFET), for example. FET 112 includes a gate connected to LO-port 110 for receiving the LO signal and a source connected to ground.
The mixer 100 also incorporates a diplexer 115, which performs frequency separation to enable RF and IF signals to be received and sent on different frequencies. The diplexer includes capacitor C1 connected to RF-port 120 and inductor L1 connected to IF-port 130. The mixer 100 is generally capable of low conversion loss and a low noise figure over its frequency range, and requires little LO drive power. Isolation of LO-port 110 is provided by the mixing FET 112. However, FET 112 leaks LO energy at frequencies where parasitic capacitance between the gate and drain (Cgd) of FET 112 is significant. The LO-port isolation may therefore not be sufficient for system requirements.
Efforts to improve LO-port isolation (and to reduce LO energy leakage) have included the addition of baluns, which isolate a single, unbalanced input line and provide a corresponding balanced output, consisting of two output lines carrying out of phase signals. For example, FIG. 2 is a block diagram of conventional mixer 200, which includes balun 216 on LO-port 110 and balun 218 on IF-port 130. FIG. 3 is a block diagram of another conventional mixer 300, which includes balun 316 on RF-port 120 and balun 318 on IF-port 130. For each balun, an unbalanced signal is carried on two signal lines, one of which is tied to ground, and the balanced signal may be carried on three signal lines, one of which is tied to ground via a center tap (not shown) and the remaining two of which carry electrical signals having equal amplitudes, but opposite phases.
Although LO-port isolation may be improved by the configurations of mixers 200 and 300, the baluns 216, 218 or the baluns 316, 318 respectively increase cost and size of the circuit, and cause additional loss. Further, when balun 316 is used on RF-port 120, the noise figure degrades in a down converter application. Therefore, balun 316 usually must be realized as a passive structure, in an attempt to minimize impact on the noise figure. Also, when balun 316 is used on RF-port 120 or balun 218 and/or 318 is used on IF-port 130, conversion loss is elevated. For example, with respect to IF-port 130, the low frequencies usually require that baluns 218, 318 be realized as coil-core transformers, which are typically relatively large and expensive.